Life history data on P. oreas and three races of P. maniculatus were gathered in the course of a study of the systematics and ecology of these mice. Five age groups were recognized, based on the amount of wear on the molars. Breeding began in late winter and was at its height in the spring. It declined during the summer and ended in late summer or early fall. P. maniculatus continued to breed for a month or more after P. oreas stopped breeding. Animals born early in the breeding season were breeding by June of the same year, but those born late in the season did not breed until the following year. Litter size averaged 6.1 in P. oreas , 5.5 in P. maniculatus . In June one-third of the catch consisted of young of the year, in July one-half, and in August three-fourths, but the proportion of young apparently declined slightly in September. Young males outnumbered young females, but old females outnumbered old males, suggesting that females live longer.